Isoolefin-conjugated diene copolymers have been known in the art for some time. Commercially these products are termed butyl rubbers. Although earlier disclosures talk of unsaturation ranges of 0.5 to 15%, commercial products were limited to the 0.5 to 4% unsaturation range. The higher unsaturation products were either low in molecular weight or resinous due to crosslinking.
More recently high number average molecular weight, high unsaturation gel-free polymers of isoolefins and conjugated dienes have been prepared by Thaler et al.; see, for example, U.S. Pat. No. 3,808.177 and U.S. application Ser. No. 348,678, now U.S. Pat. No. 3,856,763, both incorporated herein by reference. The novel aspects of the process for producing these unique polymers are (1) carrying out the reaction in a homogeneous phase; (2) using particular Friedel-Crafts catalyst systems; (3) at polymerization temperatures below -100.degree. C; and (4) using a minimum amount of nonreactive diluent.
The diluent, which is termed a cosolvent since it, together with the monomers, make up the reaction medium solvent, is used in amounts of about 5 to about 40 volume percent of cosolvent based on the cosolvent plus total monomers. Thaler et al have found that in certain instances the preferred solvents for carrying out their polymerization are methylcyclohexane and carbon disulfide.
The use of carbon disulfide can be disadvantageous in view of its toxicity and flammability. On the other hand, methylcyclohexane is a high cost solvent. Methylcyclohexane also has a normal boiling point above 100.degree. C, making it somewhat difficult to steam strip in the polymer recovery stage.
Hence, there is need for a solvent which is useful in carrying out the polymerization process which is effective as well as low in cost, toxicity and handling hazards. The cosolvent mixtures of the instant invention have a boiling point less than 100.degree. C, thus allowing said cosolvent to be easily steam stripped in the polymer recovery step.